Comparative chemical composition and antioxidant activity of olive leaves Olea europaea L. of Tunisian and Algerian varieties

The main objective of this research was to understand the effect of different solvents (ethanol, ethyl acetate, and hexane) on the extraction of bioactive compounds in addition to the antioxidant activity of eight Tunisian and Algerian olive leaf (Olea europaea L) cultivars. The different extracts were evaluated for their content of total phenols, flavonoids, and condensed tannins. The antioxidant capacity was determined by ABTS (2,2-azinobis (3 ethylbenzothiazoline-6-sulfonic acid) and DPPH (2,2-diphenyl-1 picrylhydrazyl) methods. In addition, qualitative and quantitative analyses of phenolic compounds by HPLC were performed. In terms of phenolic content, ethanol was found to be the best solvent to extract phenolic compounds with antioxidant capacity. The ethanolic extract of Sigoise and Rougette varieties were the richest in total phenols 161.54 +/- 0.99, and 160.53 +/- 1.17 mg GAE/g DE respectively. The highest levels of flavonoids were detected in ethyl acetate extracts of Rougette (87.58 +/- 6.99 mg QE/g DE) and Verdal varieties (86.77 +/- 1.75 mg QE/g DE), whereas the hexanolic extracts of Gerboui, Sofiana, and Chemlali exhibited the higher amount of tannins (67.99 +/- 5.41, 65.52 +/- 1.94, 64.64 +/- 1.51 mg CE/g DE, respectively). Furthermore, in the DPPH radical scavenging activity assay, the Sofiana extract exhibited the highest antioxidant activity (23.85 +/- 0.58 mg/L). In another case, for the ABTS radical scavenging assay, the Verdal extract showed high antioxidant potential (30.22 +/- 1.35 mg/L). HPLC analysis of ethanolic extracts of the different varieties allowed the identification of 19 organic compounds, with oleuropein (2604.01-7991.14 mg/100g DM) being the most abundant, followed by hydroxytyrosol (250.74-687.54 mg/100g DM) and verbascoside (216.12-672.64 mg/100g DM) being the major compounds in the olive leaves in all studied ethanolic extracts, with concentrations varying significantly according to the cultivars and geographical origin. Principal component analysis (PCA) performed on phenolic profiles and antioxidant activities revealed the influence of bioactive compounds on antioxidant activity, with high concentrations of phenols and flavonoids in olive leaf extracts having a strong antioxidant activity.